In recent years, demands for high strength steel sheets have been growing with the increasing needs mainly for the weight reduction of automobile bodies and the assurance of the safety of passengers in a collision. In particular, the application of steels of TS 590 MPa class (60 kgf/mm2 class) in tensile strength has rapidly expanded.
As a steel sheet used for such application, a multi-phase steel sheet comprising retained austenite and/or martensite is widely known. For example, as Japanese Unexamined Patent Publication No. H9-104947 discloses, a steel sheet having an excellent balance between strength and elongation (a total elongation is 33.8 to 40.5% when a tensile strength is 60 to 69 kgf/mm2) is obtained by containing retained austenite in an appropriate quantity therein. In this technology, however, a technology regarding the balance between strength and hole expandability has not been sufficiently considered and, in particular, technological requirements for ultra-low P, the control of the maximum length of a microstructure and inclusions and the control of the hardness of a microstructure are not, in the least, taken into consideration. Therefore, the properties of the steel sheet have been inferior (a hole expansion ratio d/d0 is 1.46 to 1.68, namely 46 to 68% in terms of a net hole expansion rate, when a tensile strength is 60 to 69 kgf/mm2) and the application has been limited.
In the meantime, Japanese Unexamined Patent Publication No. H3-180426 discloses a bainite sheet steel excellent in the balance between strength and hole expandability (a hole expansion ratio d/d0 is 1.72 to 2.02, namely 72 to 102% in terms of a net hole expansion rate, when a tensile strength is 60 to 67 kgf/mm2). However, since this technology provides not a multi-phase structure but the equalization of a structure (a bainite single phase structure), as a means of improving the net hole expansion rate, the balance between strength and elongation is rather insufficient (a total elongation is 27 to 30% when a tensile strength is 60 to 67 kgf/mm2) and the application is again limited.
That is, though, in the press forming of auto parts, punch stretch formability represented by the balance between strength and elongation and stretch flange formability represented by the balance between strength and hole expandability are two major components of forming, such a technology, satisfying both the components simultaneously, has not been available and the excellence in both has been the key to the expansion of the application.
In recent years while the shift to high strength steel sheets is progressing at an increasing rate due to global environmental issues, as their application to components with high degree of forming difficulty has been taken into consideration, a steel sheet excellent in both the balance between strength and elongation and the balance between strength and hole expandability, in other words, a multi-phase steel sheet excellent in the balance between strength and hole expandability, has been demanded.